The present invention relates to a rotary tube, in particular for a rotary tubular kiln for the production of activated carbon, as disclosed herein and to a rotary tubular kiln comprising a rotary tube of this type. Furthermore, the present invention relates to the use of this rotary tube or rotary tubular kiln for the production of activated carbon. Finally, the present invention relates to a process of producing activated carbon using this rotary tube and this rotary tubular kiln, respectively.
On account of its highly non-specific adsorptive properties, activated carbon is the most widely used adsorbent. Statutory requirements as well as increasing environmental awareness are leading to an increasing demand for activated carbon.
The activated carbon is increasingly used in both the civil and military sectors. In the civil sector, the activated carbon is used, for example, for the purification of gases, filter installations for air conditioning, automobile filters etc., while in the military sector the activated carbon is used in all kinds of protective materials (e.g. breathing masks or gas-masks, protective covers as well as all kinds of items of protective clothing, such as for example protective suits etc.).
Activated carbon is generally obtained by carbonization (which is a synonym for “pyrolysis”, “smouldering” or “coking”) and subsequent activation of suitable carbon-containing starting materials. Carbonaceous starting materials which lead to economically viable yields are preferred, since the weight losses caused by the removal of volatile constituents during carbonization and caused by burn-off during activation are considerable. For further details as to the production of activated carbon, reference can be made, for example, to H. v. Kienle and E. Bäder, Aktivkohle und ihre industrielle Anwendung [Activated Carbon and its Industrial Use], Enke Verlag Stuttgart, 1980.
The condition of the activated carbon produced—finely porous or coarser porous, strong or brittle etc.—depends on the carbon-containing starting material. Examples of standard carbonaceous starting materials include coconut shells, wood waste, peat, hard coal, pitches, but also particular plastics, such as for example sulphonated polymers, which play a major role inter alia in the production of activated carbon in the form of small granules or spheres.
Various forms of activated carbon are used: powder coal, splint coal, granular coal, shaped coal and, since the end of the 1970s, also activated carbon in granule and sphere form (known as “granular carbon” or “spherical carbon”). Activated carbon in granule form, in particular in sphere form, has a number of advantages over other forms of activated carbon, such as powder coal, splint coal and the like, making it valuable or even indispensable for certain applications: it is free-flowing, hugely abrasion-resistant and dust-free and very hard. Granular coal, in particular spherical coal, is in very great demand for particular application areas, such as for example surface filter materials for protective suits providing protection against chemical toxins or filters for low pollutant concentrations in large quantities of air, on account of its specific form and also on account of its extremely high resistance to abrasion.
In most cases, suitable polymers are used as starting materials in the production of activated carbon, in particular granular carbon and spherical carbon. Sulphonated polymers, in particular sulphonated divinylbenzene-crosslinked styrene polymers, are preferably used, in which case the sulphonation can also be achieved in situ in the presence of sulphuric acid or oleum. Examples of suitable starting materials include ion-exchange resins or precursors thereof, which are generally divinylbenzene-crosslinked polystyrene resins; in the case of the finished ion-exchangers, the sulphonic acid groups are already present in the material, and in the case of the ion-exchanger precursors, the sulphonic acid groups are still to be introduced by sulphonation. The sulphonic acid groups have a crucial function, since they perform the role of a crosslinker by being cleaved off during the carbonization. However, the large quantities of sulphur dioxide released and the associated corrosion problems, inter alia, in the production equipment represent disadvantages and problems.
Activated carbon is usually produced in rotary tubular kilns (i.e. rotary tubular furnaces). These have, for example, a location for the raw material charge to be introduced at the start of the kiln and a location for the end product to be discharged at the end of the kiln.
In the conventional processes for producing activated carbon in accordance with the prior art, both the carbonization and the subsequent activation are carried out in a single rotary tube during discontinuous production.
During the carbonization, which may be preceded by a pre-carbonization phase, the carbon-containing starting material is converted into carbon, i.e. in other words the carbonaceous (i.e. carbon-containing) starting material is carbonized. During carbonization e.g. of the abovementioned organic polymers based on styrene and divinylbenzene, which contain crosslinking functional chemical groups which on thermal decomposition lead to free radicals and therefore to crosslinking, in particular sulphonic acid groups, the functional chemical groups, in particular sulphonic acid groups, are broken down—with volatile constituents, such as in particular SO2, being released—and free radicals are formed, with a strong crosslinking action; without these free radicals, no pyrolysis residue (═carbon) would result. Suitable starting polymers of the abovementioned type include in particular ion-exchange resins (e.g. cationexchange resins or acidic ion-exchange resins, preferably comprising sulphonic acid groups, such as for example cation-exchange resins based on sulphonated styrene/divinylbenzene copolymers) or the precursors thereof (i.e. the unsulphonated ion-exchange resins, which have to be sulphonated before or during carbonization by a suitable sulphonating agent, such as for example sulphuric acid and/or oleum). The pyrolysis is generally carried out under an inert atmosphere (e.g. nitrogen) or at most a slightly oxidizing atmosphere. It may equally be advantageous to add a relatively small quantity of oxygen, in particular in the form of air (e.g. 1 to 5%) to the inert atmosphere during the carbonization, in particular at relatively high temperatures (e.g. in the range from approximately 500° C. to 650° C.), in order to oxidize the carbonized polymer skeleton and in this way to facilitate the subsequent activation.
On account of the acidic reaction products which are released during the carbonization (e.g. SO2), this stage of the activated carbon production process is extremely corrosive with respect to the kiln or furnace material and imposes extremely high demands on the corrosion-resistance of the rotary tubular kiln material.
The carbonization is then followed by the activation of the carbonized starting material. The basic principle of the activation consists in some of the carbon generated during the carbonization being selectively and deliberately broken down or burnt off selectively under suitable conditions. This forms a large number of pores, gaps and cracks, and the mass-based specific surface area (in particular the BET area) of the activated carbon increases considerably. During the activation, therefore, carbon is burnt off in a selective and controlled way. Since carbon is broken down or burnt off during the activation, this operation leads to an in some cases considerable loss of substance, which under optimum conditions equates to an increase in the porosity and means an increase in the internal surface area (pore volume, BET) of the activated carbon. Consequently, the activation is carried out under selectively or controlled oxidizing conditions. Standard activation gases are generally oxygen, in particular in the form of air, and/or steam and/or carbon dioxide as well as mixtures of these activation gases. If appropriate, inert gases (e.g. nitrogen) may be added to the activation gases. To achieve a reaction rate which is sufficiently high for industrial purposes, the activation is generally carried out at relatively high temperatures, in particular in the temperature range from 700° C. to 1,200° C., preferably 800° C. to 1,100° C. This imposes high demands on the thermal stability of the rotary tubular kiln material.
Since the rotary tubular kiln material therefore, on the one hand, has to withstand the very corrosive conditions of the carbonization phase and also, on the other hand, the high-temperature conditions of the activation phase, only materials which have a good high-temperature corrosion resistance are used for the production of the rotary tubular kiln, i.e. in particular steels, which combine a good resistance to chemically aggressive materials, in particular a good corrosion resistance, and a good high-temperature stability in a single material. These are in particular high-alloy steels, i.e. steels comprising more than 5% of alloying elements. In particular, high-alloy chromium or chromium/nickel steels are used as materials for the production of appropriate rotary tubular kilns.
However, the high-temperature corrosion-resistance steels have the critical disadvantage of having only moderate to poor welding properties. This constitutes a problem because mixing elements in the form of paddle-like circulating or turning plates—also referred to synonymously as “material guide plates”—have to be present in the interior of the rotary tubular kiln for the purpose of intimately mixing the charge material, in particular homogeneously contacting the charge material with the activation gases, and these mixing elements likewise consist of high-temperature corrosion-resistant steel and have to be welded to the inner side of the inner walls of the rotary tube in order to stably bond them to the interior of the rotary tube. Embrittlement of the material may occur during welding of these high-temperature-resistant and corrosion-resistant steels with high chromium or chromium/nickel contents (e.g. as a result of precipitation phenomena and what is known as sigma phase formation). Moreover, grain growth which is likewise associated with embrittlement of the material may occur at temperatures of over 900° C. Therefore, the welding of the mixing or circulation elements to the inner walls of the rotary tube is not without problems in conventional production apparatus.
Furthermore, during the activated carbon production process, the welds or weld seams in the interior of the rotary tube are always exposed to high levels of stress, specifically on the one hand as a result of corrosive processes during carbonization and on the other hand as a result of the very high temperatures used during activation, so that high stress and demands are applied to the weld seams. This requires constant maintenance and inspection, which is not without problems, since the weld seams are arranged on the inner side, and consequently maintenance is only possible when the rotary tube is not operating.
To avoid the problems which have been outlined above, WO 01/83368 A1 proposes that the corrosive process stage of carbonization, which is associated with the release of acidic gases (e.g. SO2), be separated from the high-temperature stage of post-carbonization and activation, i.e. that the corrosive phase of the carbonization be carried out in different equipment from the high-temperature phase of the post-carbonization and activation. Although this has the advantage that different rotary tube materials can be used for the corrosive phase of the carbonization, on the one hand, and the high-temperature phase of the post-carbonization and activation, on the other hand, with these materials respectively adapted to the corrosive phase and to the high-temperature phase, it has the associated disadvantage that carbonization and activation have to be carried out separately, i.e. cannot be carried out in a single piece of equipment in a single, discontinuous process.
Therefore, it is an object of the present invention to provide an apparatus which is suitable in particular for the production of activated carbon.
Therefore, a further object of the present invention is to provide a rotary tube and a rotary tubular kiln, in particular for the production of activated carbon, which at least partially avoids or at least palliates the disadvantages of the prior art which have been outlined above.
To solve the problem outlined above, the present invention proposes a rotary tube as disclosed herein. Further advantageous configurations and features of the present invention are also disclosed herein.
The present invention also relates to a rotary tubular kiln, which rotary tubular kiln comprises the rotary tube according to the present invention.
Furthermore, the present invention also relates to the use of the rotary tube and of the rotary tubular kiln according to the invention for the production of activated carbon.
Finally, the present invention relates to a process of producing activated carbon using the rotary tube and the rotary tubular kiln of the present invention, respectively.
Consequently, the subject-matter of the present invention—according to a first aspect of the present invention—is a rotary tube, in particular for a rotary tubular kiln for the production of activated carbon, having a plurality of mixing elements, said mixing elements being arranged in the interior of the rotary tube for circulating and/or mixing the charge material, in particular in the form of circulation or turning plates (also referred to synonymously as “material guide plates”), in which the rotary tube has apertures for receiving securing sections of the mixing elements, and the securing sections are welded to the outer side of the rotary tube. One particular feature of the present invention must therefore be considered to reside in the welding of the securing portions of the mixing elements to the outer side of the rotary tube.
The securing portions of the mixing elements are therefore fitted through the apertures in the rotary tube wall and welded to the outer side of the rotary tube. This in particular prevents the welding locations or weld seams being exposed to the aggressive conditions in the interior of the rotary tube in the operating state during production of activated carbon, i.e. corrosive gases during carbonization and high temperatures during activation. On account of the fact that the welding locations or weld seams are in this way exposed to a considerably reduced level of stress and demands, the service-life thereof is significantly increased. The welding to the outer side moreover significantly facilitates maintenance: the welding locations between mixing elements/rotary tube can readily be inspected from the outside and maintained and improved and repaired if necessary. Consequently, maintenance can be carried out even while the rotary tube is operating.
Moreover, it is in this way possible to use welding materials (also referred to synonymously as “weld materials” or “weld metal”) which ensure an optimum and tight mixing element/rotary tube joint but would not otherwise be readily able to withstand the corrosive high-temperature conditions which prevail in the interior of the rotary tube during operating in the long term.